1. Field of the Invention
This invention is in the field of chemical or ion exchange treatment of a silicate glass ophthalmic lens to develop strength by compressively stressing a surface layer of the glass. Potassium ions are introduced into the surface layer in exchange for sodium ions during the ion exchange treatment.
2. Description of the Prior Art
It is known that glass containing sodium ions can be strengthened by having contact with a molten salt containing alkali metal ions with a diameter greater than that of the sodium ions in the glass. Where potassium ions are substituted for the sodium ions, a compressive stress is developed in the surface layer of the glass article as disclosed in the Journal of the American Ceramic Society, Vol. 45, No. 2 (February 1962) pages 59-68. In the process described in the above article, ion exchange was conducted at a temperature below the strain point of the glass so as to inhibit molecular re-arrangement and viscous flow during ion exchange of the monovalent metal ions migrating into the glass surface. The larger ions from the molten salt in effect are squeezed into sites originally occupied by the smaller alkali metal ions. The compressive stress set up by this crowding effect substantially increases the impact strength of the glass.
In an article entitled "Strengthening by Ion Exchange" in the Journal of the American Ceramic Society, Vol. 47, No. 5, May 1964, pages 215-219, glasses are described which contain substantial amounts of aluminum oxide or zirconium oxide. These glasses are said to be uniquely capable of having strength imparted thereto by an ion exchange process conducted below the strain point of the glass. Such glasses also maintain high strength subsequent to being abraded to simulate ordinary usage.
In application Ser. No. 390,742, entitled "Hardened Ophthalmic Lenses and Method of Making Same", abandoned in favor of Ser. No. 541,834, filed Jan. 17, 1975 and Ser. No. 611,383, filed Sept. 8, 1975, and assigned to the assignees of the instant invention, a process is disclosed for treating an alkali metal silicate ophthalmic glass by an ion exchange process utilizing a molten bath of potassium nitrate at a temperature ranging from 400.degree. C. to 516.degree. C. This latter temperature range is above the strain point of most ophthalmic glasses but is well below the softening point of the glass. The process has the advantage that a shorter ion exchange period is thereby made feasible.
Comparative results obtained in high temperature and low temperature ion exchange processes conducted over the same time interval indicate that the low temperature ion exchange process, that is, one conducted at a temperature below the strain point of the glass results in a glass having a stressed surface layer which is normally relatively shallow and in order to obtain deeper penetration, longer treatment times are required. In the high temperature ion exchange process, that is, one using temperatures above the strain point of the glass, a stressed layer is obtained which is normally relatively deep in comparison to the stressed layers obtained by the low temperature ion exchange process. Presumably because molecular re-arrangement can take place, lower compressive stresses are generally obtained in the stressed layer of the glass.
U.S. Pat. No. 3,790,260 discusses the importance of depth of penetration of a compressively-stressed surface layer, or neutral zone, as a means of providing a satisfactory ophthalmic lens with resistance to lens breakage even after abrasion in use. The recommended depth of compression is about 100 microns or greater. Typically, the ophthalmic lens industry has employed soda-lime-silica glasses for the production of ophthalmic lenses in which this depth of compression is generally less than 60 microns. Thus, it is desirable to develop a reliable method of deepening the compressively-stressed surface layer obtained by ion exchange strengthening of an ophthalmic lens. By the present invention, an ophthalmic lens of superior strength is obtained utilizing a temperature either above or below the glass strain point.
It is also desirable to provide a compressive stress value at the surface of an ophthalmic glass lens of at least about 15,000 psi. The present invention fulfills this need and has provided various other advantages as will become apparent from the following detailed description of the invention.